136 



PLANT SOCIOLOGY 



Table 11. Mean Daily Range of Saturation Deficit, June, 1901, at 



Pawlowsk (59.41 N. Lat.) 



(After Szymkiewicz) 



If we compare the lowest values of the dryest months, we find that 

 the polar regions and mountains have the lowest saturation deficit, and 

 steppes and deserts the highest. Hilleh in Mesopotamia, during the 

 period 1908-1910, had a mean deficit of 59.3 mm., measured at 2 

 p.m. in the driest months. The Schneekoppe in the Riesen-Gebirge 

 (1,603 m.) during the period 1895-1904 had only 1.7 mm. deficit, 

 measured at the same time of day in the driest months (Szymkiewicz). 



Saturation Deficit in the Habitat of Plant Communities. — Szafer 

 et al. (1923) have studied the saturation deficits for a series of associa- 

 tions in the Polish Tatra. They found very constant and character- 

 istic differences, not only between different associations but also 

 between the different layers of vegetation. Simultaneous measure- 

 ments in a spruce wood with undergrowth of Vaccinium and in a 

 Sphagnotiim only 6 m. away gave: 



Table 12. — Saturation Deficit 

 (In millimeters) 



The air temperature in both places was practically the same. 

 Figure 73 gives a graph of the saturation deficits in two alternating 

 associations of the crest of the Siwa pass in the Pohsh Tatra. All the 

 wind-swept western slopes bear the drought-resistant Juncetum trifidi. 

 On the protected eastern slopes the Vaccinietum myrtilli prospers; 

 and on the bottoms of the depressions the Polytrichetum sexangularis 

 spreads its velvety green carpet. The saturation deficit 3 cm. above 

 the ground during a strong wind was 2.6 mm. in the habitat of the 

 Juncetum but only 1.1 mm. in the Polytrichetum. 



Miiller (1922) measured the saturation deficit in various habitats 

 of the vegetation on eroded chalk. 



